1. Field of the Invention
The present invention relates to a motor driven by a sensorless driving circuit.
2. Description of the Related Art
Conventionally, a brushless motor has been driven by a pulse width modulation (PWM) method using an inverter mainly for achieving a wide range of variable speed control and reduction in current consumption. A brushless motor with a 3-phase coil, for example, has position sensors (such as Hall elements) arranged at an interval of 120 electrical degrees to detect a rotor magnetic pole position. Based on a signal corresponding to the magnetic pole position obtained by those position sensors, the brushless motor executes a near-sinusoidal wave driving by operating each switching element in the inverter device.
Further, diverse sensorless driving techniques have been developed for reducing the cost and size of a brushless motor by omitting position sensors. One way to achieve such a sensorless driving employs a 120-degree conduction scheme or a wide angle (less than 180 degrees) conduction scheme to detect a zero-crossing point of an electromotive force generated during a non-conducting period. Therefore, in these conduction schemes, each phase is required to have a non-conducting period for detecting a magnetic pole position. Unfortunately though, the existence of the non-conducting period may cause vibration and noise at the timing of commutation.
In view of this, a near-sinusoidal wave sensorless driving recently has been developed to reduce vibration and noise by eliminating the necessity of the non-conducting period (one example of the near-sinusoidal wave sensorless driving that does not require a non-conducting period is disclosed in Japanese Patent Application Publication No. 2006-230120).
In the above technique, the near-sinusoidal wave sensorless driving makes use of a superimposed voltage of 3Nth (where N is a positive integer) harmonic components contained in an induced electromotive force to detect a magnetic pole position without introducing the non-conducting period. Therefore, if the induced electromotive force is reduced by a low-speed rotation, or if an amplitude ratio of the 3Nth harmonic components to the fundamental wave component is extremely small, it is difficult to detect a magnetic pole position with a high accuracy.